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ThunderbirdApplication · Mozilla

CVE-2025-5986

MEDIUM · 6.5 CVSS v3.1 Published 2025-06-11
Fix available
A fix is available. Upgrade to 128.11.1 / 139.0.2 or later.
See remediation →
72/100
Remediation priority · Elevated
Remotely reachable No privileges

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
A crafted HTML email using mailbox:/// links can trigger automatic, unsolicited downloads of .pdf files to the user's desktop or home directory without prompting, even if auto-saving is disabled. This behavior can be abused to fill the disk with garbage data (e.g. using /dev/urandom on Linux) or to leak Windows credentials via SMB links when the email is viewed in HTML mode. While user interaction is required to download the .pdf file, visual obfuscation can conceal the download trigger. Viewing the email in HTML mode is enough to load external content. This vulnerability was fixed in Thunderbird 128.11.1 and Thunderbird 139.0.2.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Thunderbird fails to properly restrict mailbox:/// links in HTML emails, allowing crafted messages to trigger automatic, unsolicited PDF downloads to the user's desktop or home directory even when auto-saving is disabled. This can be exploited for disk space exhaustion or Windows credential leakage via SMB links.

MitigationUpdate Thunderbird to version 128.11.1 or 139.0.2 or later. Consider disabling HTML email rendering or external content loading as a defense-in-depth measure.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
ThunderbirdApplication
Affected:< 128.11.1>= 135.0, < 139.0.2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Verify Thunderbird version
    In Thunderbird, go to Help > About Thunderbird (or Thunderbird > About Thunderbird on macOS) to see the installed version number
    Affected if The installed version is before 128.11.1, or is 135.0 through 139.0.1 (the version falls within < 128.11.1 OR >= 135.0 AND < 139.0.2)
  2. Confirm HTML email rendering is enabled
    Go to Settings (or Preferences) > Privacy & Security > Display external content in messages - check if this option is enabled
    Affected if HTML email rendering or external content loading is enabled (this is the attack surface for the mailbox:/// link vulnerability)
  3. Check email account settings for content handling
    Right-click on each email account > Settings > Display > ensure 'Render HTML messages' is set to 'As plain text' or verify that 'Allow remote content in messages' is disabled
    Affected if HTML message rendering is set to render as HTML rather than plain text, and remote content is allowed

A user is affected if their Thunderbird version falls in the ranges < 128.11.1 OR >= 135.0 to < 139.0.2 AND they have HTML email rendering or external content loading enabled, as this allows the mailbox:/// link attack vector to execute.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 128.11.1 / 139.0.2 or later
Fixed in 128.11.1139.0.2
Interim mitigation

Update Thunderbird to version 128.11.1 or 139.0.2 or later. Consider disabling HTML email rendering or external content loading as a defense-in-depth measure.

Recommended fix High confidence

Thunderbird 128.11.1 (older branch) or Thunderbird 139.0.2 (newer branch)

  1. Check your current Thunderbird version by clicking Help > About Thunderbird
  2. If your version is < 128.11.1 (older branch), upgrade to version 128.11.1 or later
  3. If your version is >= 135.0 but < 139.0.2 (newer branch), upgrade to version 139.0.2 or later
  4. Download the latest Thunderbird from the official Mozilla website (www.mozilla.org) or use your system's package manager to update
  5. Restart Thunderbird after the update completes
Caveat Standard Thunderbird upgrades are generally low-risk; ensure your profile is backed up before major version jumps

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Thunderbird Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing1.0 h
  • Review / QA1.0 h
4.0 hours of engineering $750
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,200.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2025-5986 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data